[Zero]

i think - and hollywood can use this if i can have .01% of the gross - that the best villain would be ultron. set it up in Iron Man II as some massive computer program to help Tony do his thing (maybe even help track down Hulk, who will obviously be the hook to bring the heroes together). then in avengers ultron goes beserk and the heroes (and hulk) band together to stop the near unstoppable robot and his legions.

given that i also think Thor will be stupid - i hope they don't try to introduce Loki and use him as villain - (unless somehow tied to Ultron, which could actually be cool - Loki tampers with cybernetics and brings it to sentient, malevolent life - damn my ideas are so cool I don't know why Hollywood doesn't come begging for my wisdom!!!)